1. Field of the Invention
The present invention relates to a pressurized internal circulating fluidized-bed boiler, and more particularly to a pressurized internal circulating fluidized-bed boiler for use in a pressurized fluidized-bed combined-cycle electric generating system in which a fuel such as coal, petro coke, or the like is combusted in a pressurized fluidized bed and an exhaust gas produced by the combusted fuel is introduced into a gas turbine.
2. Description of the Prior Art
Efforts to reduce the emission of carbon dioxide from various sources are important in view of environmental damages that are being caused by air pollution which appears to be more and more serious on the earth. It is conjectured that coal will have to be relied upon as a major energy resource because greater dependency on nuclear and oil energies is not favorable at present. To suppress carbon dioxide emission and provide a substitute for oil and nuclear power, there has been a demand for a highly efficient, compact electric generating system which is capable of utilizing coal combustion to generate a clean energy.
To meet such a demand, atmospheric fluidized-bed boilers (AFBC) capable of burning coals of different kinds for electric generation have been developed because a stable energy supply cannot be achieved by pulverized coal boilers which pose a limitation on available coal types.
However, the atmospheric fluidized-bed boilers (AFBC) fail to perform the functions that have been expected. In addition, since only steam turbines can be combined with the atmospheric fluidized-bed boilers, there are certain limitations on attempts to increase the efficiency and energy output of the atmospheric fluidized-bed boilers. These disadvantages of the atmospheric fluidized-bed boilers have directed research and development trends toward pressurized fluidized-bed boilers (PFBC) that make it possible to construct combined-cycle electric generating systems with gas turbines.
Further, there has been researched a coal gasification combined-cycle electric generating system in which coal is converted into gas and a refined gas purified by removing dust particles is supplied to a gas turbine. The coal gasification combined-cycle electric generating system, incorporating an air-cooled gas turbine which uses exhaust gas of 1300.degree. C., has a target efficiency of 47.1% at a generating end.
On the other hand, the pressurized bubbling type fluidized-bed boilers with a capacity over 80 MWe are already in operation overseas as demonstration or commercial models and, in addition, has an advantage that a desulfurization equipment is not required. However, the coal gasification electric generating system is superior to the pressurized fluidized-bed combined-cycle electric generating system in efficiency. Thus, a topping-cycle combined electric generating system which has advantages of both systems and higher efficiency has been researched.
The topping-cycle combined electric generating system, comprises a gasifier in which coal is decomposed into coal gas and char, and an oxidizer comprising a fluidized-bed boiler in which char produced in the gasifier is combusted. The coal gas produced in the gasifier and the exhausted gas produced by combustion of char in the oxidizer are mixed and combusted at the inlet of a gas turbine to thereby produce high-temperature gas. The produced high-temperature gas is then supplied to the gas turbine which drives an electric generator coupled thereto.
Conventionally, the pressurized fluidized-bed boilers with a capacity over 80 MWe which are in operation as demonstration or commercial models are made up of pressurized bubbling type fluidized-bed boilers.
However, the pressurized bubbling type fluidized-bed boiler has the following disadvantages.
(A) Disadvantage in load control
The pressurized bubbling type fluidized-bed electric generating system is controlled to meet a load imposed thereon by varying the height of the fluidized bed in the combustor. More specifically, the fluidized medium is drawn from the combustor into the storage container to change heat transfer area of the heat transfer tube, thereby controlling the steam generation to meet the load. When the heat transfer surfaces of the heat transfer tube are exposed to the gas, the heat transfer coefficient thereof is lowered, and hence the amount of heat recovered is lowered. Since the exhaust gas emitted from the fluidized bed is cooled by the exposed heat transfer surfaces, the temperature of the exhaust gas supplied to the gas turbine is lowered, thus reducing the output energy of the gas turbine.
However, the above control process is disadvantageous in that the bed material storage container is necessary to withdraw and return the high-temperature fluidized medium from and into the combustor, it is not easy to withdraw and return the fluidized medium at high temperature and pressure, and agglomeration tends to occur when the fluidized medium of high heat capacity are taken into and out of the bed material storage container.
Furthermore, since the pressurized fluidized-bed boiler is under pressure, the heat transfer tube in a splash zone of the fluidized bed is more subject to erode than that in the atmospheric fluidized-bed boilers (AFBC). Another problem is that an large amount of carbon monoxide is produced because an exhaust gas emitted from the fluidized bed is cooled by the heat transfer tube and the exhaust gas remains in the fluidized bed for a short period of time as the height of the fluidized bed is reduced in the time of low load.
(B) Large-sized pressure vessel
conventionally, the pressurized bubbling type fluidizedbed boiler comprises square combustors 146 accommodated in a circular pressure vessel 145 in a plan view as shown in FIG. 14. Therefore, a useless space is defined between the combustors 146 and the pressure vessel 145, resulting in a large-sized pressure vessel and increasing the construction cost of the boiler.
In other to solve the above problems, Mr. Jim Anderson of A.B.B Carbon, A.B. proposed a certain pressurized bubbling type fluidized-bed boiler in principles and design philosophy for a 350 MWe PFBC module. The pressurized bubbling type fluidized-bed boiler is constructed by combining diamond-shaped three combustors 147 to form a hexagonal profile in a plan view as shown in FIG. 15. Assemblage of the combustors 147 which is brought close to a circular shape is accommodated in a circular pressure vessel 145. A useless space between the combustors 147 and the pressure vessel 145 is reduced and the pressure vessel is downsized. The reason for the above structure is that arrangement of heat transfer tubes is complicated in the pressurized bubbling type fluidized-bed boiler having a cylindrical combustor.
Further, since the bed material storage container and pipes are necessary to withdraw and return the high-temperature fluidized medium from and into the combustor, housing of bed material storage container and the pipes inside the pressure vessel makes the pressure vessel large.
(C) Erosion of the heat transfer tube
In the conventional pressurized bubbling type fluidized-bed boiler, the heat transfer tube is more subject to erode, because the heat transfer tube is disposed in the fluidized bed where the fluidized medium is intensely fluidized. Therefore, the heat transfer tube is required to have surface treatment like thermal spraying.
(D) Complicated fuel supplying system
In the conventional pressurized bubbling type fluidized-bed boiler, fuel such as coal is insufficiently dispersed horizontally in the fluidized bed. In order to avoid nonuniform combustion, many fuel feeding pipes must be installed in the boiler, resulting in a complicated fuel supplying system. Further, it is difficult to supply fuel such as coal to each of the fuel feeding pipes uniformly. Unbalanced supply of fuel causes nonuniform combustion and generates agglomeration, resulting in shutdown of the boiler.
(E) Wear of limestone
In the conventional pressurized fluidized-bed electric generating system, limestone is mixed with the fluidized medium for desulfurization. However, the limestone wears rapidly, and is scattered as ash from the dust collector without sufficiently contributing to the desulfurizing action. The conventional pressurized fluidized-bed electric generating system fails to achieve a high desulfurization rate that are required by power plants. The conventional pressurized bubbling type fluidized-bed requires plenty of desulfurizing agent in order to obtain high desulfurization rate, and then produces a vast amount of ashes.
On the other hand, in the topping-cycle combined electric generating system, the fluidized-bed boiler which is used as an oxidizer has the same disadvantages as mentioned above.
Further, a fixed-bed gasifier is disadvantageous in that coal tar remains in the fixed bed, and an entrained flow gasifier is disadvantageous in that ash-sticks occurs because of high temperature reaction. On the contrary, a fluidized-bed gasifier has advantages that coal tar does not remain, ash-sticks does not occur and desulfurization is performed in the fluidized bed, because it is in operation at the intermediate temperature of the above two types of gasifier. However, the bubbling type fluidized-bed gasifier has the same disadvantages as enumerated in (A)-(D).